* cygprops.h: Use #pragma once.

* mkglobals_h: Ditto for generated globals.h file.
* globals.cc: Use specific NO_GLOBALS_H flag to control inclusion of globals.h.
* winsup.h: Honor NO_GLOBALS_H to control inclusion of globals.h.  Make
clear_procimptoken extern inline so that it is only defined when needed.

winsup/cygwin/mkglobals_h

@@ -14,10 +14,10 @@ s/-NL-/\n/sog;
 s/-EQ-/=/sog;
 s/\s+\n/\n/sog;
 s/\n{2,}/\n/sog;
-print <<PRELUDE,$_,"#endif /*_GLOBALS_H*/\n";
+print <<PRELUDE,$_;
 /* $target - Autogenerated from @argv.  Look there for comments. */
 
-#ifndef _GLOBALS_H
+#pragma once
 PRELUDE
 close $target_fd;
 sub munge($) {